1. Field of the Invention
This invention relates to a process for producing a photosensitive resin having a high molecular weight.
2. Related Art Statement
A polyimide having excellent heat resistance, and prominent electrical insulation properties, mechanical strength and the like has heretofore been used as a surface-protective film of a semiconductor device, an interlaminar insulating film of a semiconductor device or the like. However, in order to simplify the complicated process for preparing a polyimide pattern, attention has recently been paid to a technique for photosensitizing the polyimide per se. For example, a polyimide precursor composition which has been photosensitized by an ester group having a structure represented by the following formula or the like is known (see, for example, U.S. Pat. No. 4,040,831): ##STR5##
In all of the above cases, the photosensitized polyimide precursor composition is dissolved in a suitable organic solvent, the resulting varnish is coated on a substrate and the resulting assembly is dried, irradiated with an ultraviolet ray through a photomask, developed and then rinsed to form the desired pattern, after which the assembly having the desired pattern is further heat-treated to obtain a polyimide film.
When the photosensitized polyimide is used, in addition to such an effect that the pattern-forming step is simplified, the process is safe and low in environmental pollution because it is not necessary to use an etching solution having a strong toxicity. Therefore, the photosensitization of polyimide is expected to become a more important technique in the future.
As a process for introducing an ester group into a polyimide, there is reported a process which comprises first reacting an acid dianhydride with an alcohol compound having a photosensitive group, then converting the reaction product to an acid chloride and thereafter copolymerizing the resulting acid chloride with a diamine (see U.S. Pat. No. 4,040,831). Also, a process in which a carbodiimide is used as a condensation agent in the above condensation reaction is known (see U.S. Pat. No. 4,654,415 and Japanese Patent Application Kokai (Laid-Open) No. 61-293,204). The resin synthesized using the carbodiimide has such a benefit that the chlorine content is as small as less than several ppm, but has the following problems.
In general, carbodiimides are condensation agents which are used in the synthesis of a peptide from an amino acid. The reaction mechanism is such that the carbodiimide is first reacted with a carboxylic acid to produce an O-acylisourea, this O-acylisourea is further reacted with another molecule having carboxyl group to form a symmetric acid anhydride, and this anhydride is reacted with an amine to form an amido linkage. However, in this reaction, if a strong basic amine is present in the reaction system, the O-acylisourea is rearranged as a side reaction to produce an acylurea having no reactivity as a by-product.
When it is intended to obtain a photosensitive resin, a polyimide precursor is produced using a carbodiimide from a diamine and a tetracarboxylic acid diester having an acryl or methacryl group obtained by a reaction between an alcohol compound having a photosensitive group and an acid dianhydride; however, unlike the case where a polyamide is obtained from a usual dicarboxylic acid and a diamine, the tetracarboxylic diester having an acryl or methacryl group which is very great in steric hindrance hardly forms a symmetric anhydride and is very low in reactivity.
Also, for the above-mentioned reason, an acylurea is formed as a by-product to terminate the end of the product and hence the formation of a high molecular weight compound becomes very difficult. Therefore, in order to inhibit the side reaction, the reaction is effected at a low temperature in some methods. However, in this case, it is difficult to completely inhibit the side reaction because of heat generation due to reaction or the like and only a polyimide precursor having a low molecular weight is obtained. Also, the reaction method has been extensively studied. For example, the Journal of Chemical Society of Japan, Vol. 80, No. 12 (1959) shows on page 1497 an example in which N,N'-dicyclohexylcarbodiimide (referred to hereinafter as DCC) is used as the carbodiimide, and reports that when the acid and the DCC are allowed to react for a while and thereafter the amine is gradually added dropwise, the production of N-acylurea can be more sufficiently inhibited than when the necessary amounts of the acid, the amine and the DCC are added at one time at the beginning.
However, even if the polyimide precursor is synthesized taking the addition order into consideration (see Japanese Patent Application Kokai (Laid-Open) No. 61-293,204), actually, only a polyimide precursor having a low molecular weight is obtained. A photosensitive resin composition in which such a low molecular weight polyimide precursor is used has a low sensitivity, and a polyimide film obtained by heat-treating the same is inferior in mechanical characteristics, and hence, there is a problem in its practical use.
3. Object of the Invention
An object of this invention is to provide a process for producing a photosensitive resin having a high molecular weight, as a result of which a photosensitive resin having a high sensitivity and excellent cured film characteristics is obtained.
Other objects and advantages of this invention will become apparent from the following description.